1. Field of Invention
This invention relates to evaporative cooling units, and particularly to “multi-stage” units designed primarily to cool water, or both water and air, to temperatures lower than can be achieved in simple “direct evaporative” cooling devices.
2. Description of Related Art
Simple evaporative coolers benefit from the psychrometric process in which dry air and water can be cooled by adding moisture. At their performance limit, these coolers can cool both air and water to the outdoor wet bulb temperature. Multi-stage evaporative coolers use an indirect evaporative process to cool some of the air without adding moisture. This indirect process also lowers the wet bulb temperature of the indirectly-cooled air, making it possible in a second, direct cooling stage, to cool both air and water to a lower temperature than the wet bulb temperature of the original dry air. Additional indirect stages after the first can continue lowering the wet bulb temperature to achieve cooler and cooler “product” (air or water); the theoretical limit is the dew point temperature of the outdoor air. However, it is not practical to achieve this limit for cooling air because a great deal of “parasitic” energy would be consumed forcing air through the multiple indirect stages.
In the prior art, multi-stage evaporative cooling processes have primarily been applied to cooling air in applications where the lower outlet air temperatures (compared with a direct evaporative process) allow two-stage evaporative cooling to be substituted for a vapor-compression mechanical cooling process. One such example is the “Regenerative Evaporative Cooler” described in U.S. Pat. No. 6,338,258. This design uses alternating wet and dry heat exchange passages to cool a dry air stream, with a portion of the cooled air then supplying the wet “secondary” passages that indirectly cool the dry passages. The dry air stream can be further evaporatively cooled in a direct stage to complete the process before being delivered into a building as supply air. U.S. Pat. No. 5,301,518 describes another indirect stage that uses a portion of the indirectly cooled airstream as secondary air for the wet passages. This design features a low profile plate system that eliminates the circulation pump by wicking water from the sump to the wet plate surface. Both of these designs are intended solely to cool air in the indirect stage.
Two stage systems are seldom used to cool water. Many one-stage evaporative cooling systems called “cooling towers” are used to cool condenser water in large cooling systems. Cooling towers use fans to draw outdoor air through a distributed falling water pattern, such that the air is humidified as it cools the warm water leaving the chiller condenser. Cooler water entering the condenser increases chiller efficiency, and increasing the cooling tower size is often a cost-effective strategy for lowering the water temperature. But, simple cooling towers cannot cool water to below the outdoor air wet bulb temperature, as two stage units can. In the future, if energy costs continue to rise as expected, two stage cooling towers might achieve favorable paybacks.
A major untapped opportunity for commercial building systems is evaporative pre-cooling of ventilation air. At least 10% of supply air in many such buildings is typically outdoor air needed for building ventilation. In some cases, particularly for laboratory facilities, cooling systems must deliver 100% outdoor air. In warm weather, cooling of ventilation air represents a significant fraction of the total cooling load. In very dry climates, ventilation air can be pre-cooled by a direct evaporative process, but in most applications an indirect process that adds no moisture to the ventilation air is preferred. A plate-type indirect heat exchanger used as a booster stage for a cooling tower could also be used to pre-cool ventilation air.
Another ventilation air cooling opportunity is for “dedicated outdoor air” units that detach the ventilation air load from other HVAC components. These dedicated units are receiving increasing attention as an option to “variable-air-volume” (VAV) systems that have difficulty maintaining required fresh air volumes at low speeds. A plate-type heat exchanger delivering 100% outdoor air, with building exhaust air used in alternating wet passages, can be used as an indirect evaporative ventilation air cooling unit in the cooling season and, without water feed to the exhaust air passages, as a heat recovery unit in the heating season. Most current forced air heating systems fail to take advantage of the opportunity to apply heat exchangers for pre-heating ventilation air from warm building exhaust air.
Most new low-rise non-residential buildings in the U.S. are cooled by packaged rooftop units (“RTU's”) that include one or more compressors, a condenser section that includes one or more air-cooled condensing coils and condenser fans, an evaporator coil, a supply blower, an intake location for outdoor ventilation air (with or without an “economizer” to fully cool from outdoor air when possible), optional exhaust air components, and controls. These components are packaged by manufacturers in similar configurations that, because they are air-cooled, fail to take advantage of the opportunity to improve efficiency and reduce electrical demand through evaporative cooling of both condenser and ventilation air streams. This opportunity is particularly significant in dry climate locations where rapid growth and focus on low construction costs have caused a high percentage of non-residential cooling systems to use RTU's rather than more efficient systems that use chillers and cooling towers.
There is also an opportunity for energy-efficient systems that can deliver “naturally-cooled” water for circulation through tubing in concrete slabs to pre-cool the building structure. The tubing can function reversibly to deliver comfortable radiant floor heating in winter.
For these and other reasons, there is a need for improved cooling units that incorporate plate-type evaporative heat exchangers that efficiently cool either water or air, or both, to temperatures lower than can be achieved in conventional evaporative coolers.